Media captionWilliam Hague: "Let's make full use of Sri Lanka being in the spotlight"

The UK would not be helping Sri Lanka if it joined a boycott of a Commonwealth summit there this week, Foreign Secretary William Hague says.

The Canadian and Indian prime ministers are expected to miss the event over concerns about human rights abuses.

David Cameron said there were "serious questions" about war crimes allegedly committed during Sri Lanka's civil war.

Mr Hague said the UK could raise this matter and other serious issues only "if we are there".

Mr Hague told the BBC he understood why other leaders had chosen not to attend, but for the UK to join them would "damage the Commonwealth without changing things positively in Sri Lanka".

The prime minister has said he will urge Sri Lanka President Mahinda Rajapaksa to hold an inquiry into the final months of the Sri Lankan civil war when they meet during the summit, which takes place from Friday to Sunday.

'In the spotlight'

Critics say any engagement with the Sri Lankan government at the Commonwealth Heads of Government Meeting (Chogm) amounts to "collaboration".

The UN estimates that 40,000 civilians were killed in the last five months of the 26-year conflict, which ended in 2009, but the Sri Lankan authorities deny responsibility for any such events.

The BBC Our World programme has obtained evidence that Sri Lankan government security forces have been involved in the torture and rape of Tamil civilians as recently as this year.

Mr Hague, who will accompany Mr Cameron to Sri Lanka, said they would do all they could to address the issue of human rights when they visited the country and "we can only do that if we are there".

"We are going to say 'Sri Lanka is in the spotlight so let's make full use of it being in the spotlight'," he told the BBC's Andrew Marr programme.

"The prime minister is going to visit the north. He is the first head of government from any country since Sri Lankan independence in 1948 to visit the north. I shall be visiting places to promote reconciliation.

"It will make more impact in Sri Lanka with the prime minister and me doing that than sitting in our offices in London."

'Immense suffering'

Mr Cameron said recent "positive steps" in the country, such as provincial elections and a commission to investigate the disappearances of tens of thousands of people, fell well short of what was required.

He said a documentary commissioned by Channel 4 about the events of 2009, which features footage of alleged war crimes by government forces, was "one of the most chilling documentaries" he had ever watched.

The Channel 4 film recorded the apparent indiscriminate shelling of areas packed with civilians, including hospitals, extrajudicial executions and rapes by soldiers.

If the prime minister now chose to reverse his decision to attend the summit he would have Labour's full support.Douglas Alexander, Shadow foreign secretary

"It brings home the brutal end to the civil war and the immense suffering of thousands of innocent civilians who kept hoping that they would reach safety, but tragically many did not," the prime minister said.

While he said no-one could regret the end of the campaign waged by the Tamil Tigers, who were responsible for "terrible crimes", the documentary raised "very serious questions that the Sri Lankan government must answer about what it did to protect innocent civilians".

These questions, he added, "strengthen the case for an independent investigation, questions that need answers if Sri Lanka is to build the truly peaceful and inclusive future its people deserve".

"I will raise my concerns when I see President Rajapaksa next week in Colombo," he said.

"And I will tell him that if Sri Lanka doesn't deliver an independent investigation, the world will need to ensure an international investigation is carried out instead."

'Confused'

Canada's Prime Minister Stephen Harper has said he will boycott the meeting and India's prime minister Dr Manmohan Singh is also expected to stay away, although the country's foreign minister will attend.

Labour have said the UK's position is "confused" and the government should consider, even at this late stage, "downgrading" the status of the delegation it will send.

"For months Labour has urged the government to do more to raise Britain's concern over human rights in Sri Lanka in the run up to the summit," said shadow foreign secretary Douglas Alexander.

"If the prime minister now chose to reverse his decision to attend the summit he would have Labour's full support."

The Prince of Wales will be attending to represent the Queen, who is head of the Commonwealth.

UN High Commissioner for Human Rights Navi Pillay has warned Sri Lanka - considered a "country of concern" by the Foreign Office - is heading in an "increasingly authoritarian direction".

The Sri Lankan government denies allegations that it has been complicit in kidnappings, torture and other abuses amid mounting concerns over "disappearances" and attacks on the judiciary and press.